The present invention relates to an apparatus and method for cutting a wellbore particularly but not exclusively for accessing sub-surface hydrocarbon bearing earth formations.
In the oil and gas production industry wellbores are drilled from the earth's surface to access sub-surface hydrocarbon bearing formations. The wellbores are typically lined with a string of metal tubulars known as casing. The exterior of the casing is typically held in place in the wellbore using a bonding material such as cement.
It is usually the case that a drill and drill string is used to drill the initial wellbore. A ‘trip out’ process is then required where the drill and drill string are removed from the wellbore and the casing is subsequently run into the wellbore. A reamer shoe may be positioned at the lowermost end of the casing so as to ream the drilled wellbore as the casing is run in. The reamer shoe may be driven by rotating the casing itself, if this is possible, or may be driven by a motor at the bottom of the casing. In the latter case, the reamer shoe and motor need to subsequently be removed prior to further sections of casing being inserted. Further sections of casing are usually required for deeper wellbore sections, and these are run in through the initial section of casing after a further drilling process has occurred, the further drilling process requiring a drill and drill string to be run in through the initial section of casing.
Likewise when drilling an initial wellbore it can be desirable to be able to use multiple drilling operations in as quick succession as possible.
The repeated cycles of drilling and/or reaming steps are time consuming particularly in terms of the trip out and run in processes associated with removing and reinserting the drill and drill string and/or reamer shoes and associated motors.
Our earlier international patent application PCT/GB2007/002874 discloses a solution to these problems wherein the reamer shoe and a motor are positioned at the lower end of the casing and have sacrificial components that are arranged to be readily drilled out using a drill and drill string subsequently inserted into the casing. This removes the necessity for a trip out process associated with the reamer shoe and motor prior to subsequent drilling.
Whilst the apparatus and method described in our above mentioned earlier international application provide significant time and therefore cost savings over previous proposals, there is a need to further reduce the time taken, as the process wherein the motor and reamer shoe are drilled through may still take longer than desired.